Was the stuffing precooked or requires full cooking? It would largely depend on this factor. If the stuffing was cooked it would require very short cooking time, the fillet of fish cook quickly in the oven, especially if it is sliced thin, and the stuffing inside only needed to be heated through.
If it was uncooked it would be more tricky, probably needed to be wrapped in foil, but if it was a ready made product from a supermarket, I am pretty sure they were precooked... is there anyway you can check on that to make sure?

Because it was frozen (actually, even if it wasn't), it should be cooked until the internal temp. reaches at least 140-145 to prevent any food borne illness, regardless of if the stuffing was cooked or not. One could take a chance and just cook it until the stuffing is heated through, but it would be better if it hit the 140-145 mark. This is how a lot of people get sick in places where food is normally cooked beforehand, and then reheated for service (i.e. buffets). Often times food is thrown directly into a warmer and then into a holding container heated by hot water, a sterno, or an induction burner, and it never reaches the temperature that it needs to be.

Seafood should be thoroughly cooked to an internal temperature of at least 145 F (63 C). Fish that's ground or flaked, such as a fish cake, should be cooked to at least 155 F (68 C), and stuffed fish to at least 165 F (74 C).
If you don't have a meat thermometer, there are other ways to determine whether seafood is done:

For fish, slip the point of a sharp knife into the flesh and pull aside. The edges should be opaque and the center slightly translucent with flakes beginning to separate. Let the fish stand three to four minutes to finish cooking.

For clams, mussels and oysters, watch for the point at which their shells open. Boil three to five minutes longer. Throw out those that stay closed.

When using the microwave, rotate the dish several times to ensure even cooking. Follow recommended standing times. After the standing time is completed, check the seafood in several spots with a meat thermometer to be sure the product has reached the proper temperature.

Note that there are different and higher temperature guidelines for stuffed fish (actually any stuffed meat or fish) due to the possible introduction of external microorganisms not naturally found in the meat or fish.